Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Schematic:

SYSTEM DESCRIPTION

The Evaporative Emission Control System (EECS) used on this vehicle is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon storage canister. This retains the fuel vapors when the vehicle is not operating. When the engine is running, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

Fuel Vapor Canister:

VAPOR CANISTER

The canister has an integral diaphragm operated purge valve. When the engine is running, vacuum is supplied to the top of the purge valve (Control Vacuum Signal) which lifts the valve diaphragm and opens the valve when the throttle is above a specified opening. The lower tube on the purge valve is connected to manifold vacuum at the throttle body.


EVAPORATIVE CONTROL

The system has a remote mounted purge control solenoid. The ECM controls the vacuum solenoid to the canister purge valve. Under cold engine or idle conditions, the solenoid is turned on by the ECM, which closes the solenoid and blocks the vacuum to the canister purge valve. The control valve only opens when sufficient vacuum is present.
The ECM turns off the solenoid valve and starts to purge when all of the following have been established:

1. Above a specified road speed
2. Engine is warm
3. The engine must have been running for a specified time
4. Above a specified throttle opening

Tank Pressure Control Valve:

FUEL TANK PRESSURE CONTROL VALVE

The fuel tank pressure control valve, located in the engine compartment, is a biassed spring diaphragm valve, normally closed. When the engine is running and the throttle is above a specified opening, vacuum is applied to the valve. Fuel vapors are now free to vent to the canister. When the engine is not running, no vacuum is applied to the valve, and the fuel tank is vented through the restriction valve. This valve tends to hold vapor in the tank instead of using the vapor canister.